Airborne reconnaissance systems have been widely used for many years now, particularly for obtaining images from the air of areas of interest.
Originally, a film camera was used on board aircraft for capturing the images. The main drawback of the film-camera based reconnaissance system is the long duration required for developing the film, an operation that can be performed only after landing. This problem has been overcome in more modern systems by the use of a one-dimensional vector or a two-dimensional array of light-sensitive sensors (generally called “Focal Plane Array”—FPA) in the camera for obtaining electronic images that are then electronically stored within the aircraft, and/or transmitted to a ground base station.
Airborne reconnaissance systems are generally used to obtain images of hostile areas, and therefore the task of obtaining such images involves some particular requirements, such as:
1. Flying the aircraft at high elevation and speed in order to reduce the risk of being targeted by enemy weapons, and in order to widen the area captured by each image;
2. Capturing as much as possible of relevant image information during as short as possible flight;
3. Operating under various visibility conditions, while not compromising the resolution of the images and their quality.
4. Photographing of rough terrains (e.g., high mountains, areas having sharp ground variations), in high resolution and image quality.
The need for securing the reconnaissance aircraft, while flying above or close to hostile areas has significantly increased the flying costs and risks, as sometimes the reconnaissance mission requires escorting of the aircraft by other, fighter aircrafts. Therefore, the need for enabling a short and reliable mission, while at the same time capturing as many as possible reconnaissance images of the terrain during the flight is of a very high importance.
There are several other problems involved in carrying out airborne reconnaissance. For example, the capturing of images from a fast-moving aircraft introduces the need for a so-called Forward Motion Compensation (Hereinafter, the term “Forward Motion Compensation” will be shortly referred to as FMC. Motion Compensation in general will be referred to as MC), which compensates for the aircraft movement during the opening of the camera shutter (whether mechanical or electronic; in the latter case, the opening of the camera “shutter” for the purpose of exposure is essentially the integration of light photons at the focal plane array).
The following are three examples for airborne reconnaissance systems that are typically used:                (i) The Along-Track Scanning (also known as “push-broom scanning”)—In a first configuration of the Along-Track Scanning, the light-sensitive sensors are arranged in a one-dimensional vector (row), perpendicular to the flight direction. The scanning of the imaged area is obtained by the progression of the aircraft. In one specific configuration of Along-Track Scanning, generally called Along-Track TDI (Time Delayed Integration) configuration, a plurality of such parallel one-dimensional vectors (pixel-rows) perpendicular to the flight direction are provided at the front of the camera forming a two-dimensional array. In that case, however, the first row of the array captures an area section, while all the subsequent rows are used to capture the same section, but at a delay dominated by the aircraft progression. Then, for each row of pixels, a plurality of corresponding pixels of all the rows in the array, as separately measured, are first added, and then averaged in order to determine the pixel measured light intensity value. More particularly, each pixel in the image is measured N times (N being the number of rows) and then averaged. This Along-Track TDI configuration is found to improve the signal-to-noise ratio, and to improve the image quality and the reliability of measuring.        (ii) The Across-Track Scanning (also known as “Whiskbroom Scanning”)—In the Across-Track Scanning, a one-dimensional sensing vector of light-sensitive sensors, arranged parallel to the flight direction, is used. The sensing vector is positioned on gimbals having one degree of freedom, which, during the flight, repeatedly moves the whole vector right and left in a direction perpendicular to the direction of flight, while always keeping the vector in an orientation parallel to the direction of flight. Another Across-Track Scanning configuration uses a moving mirror or prism to sweep the line of sight (hereinafter, LOS) of a fixed vector of sensors across-track, instead of moving the vector itself. In such a case, the Across-Track Scanning of the area by the gimbal having one degree of freedom, while maintaining the forward movement of the aircraft, widens the captured area. Another configuration of the Across-Track Scanning is the Across-Track TDI configuration. In this configuration there exists a plurality of vectors (columns) in a direction parallel to the flight direction, forming a two-dimensional array. This Across-Track TDI, in similarity to the Along-Track Scanning TDI, provides an improved reliability in the measuring of pixel values, more particularly, an improvement in the signal-to-noise ratio.        (iii) Digital Framing Scanning—In Digital Framing Scanning, a two-dimensional array of light-sensitive sensors is positioned with respect to the scenery. In U.S. Pat. No. 5,155,597 and U.S. Pat. No. 6,256,057 the array is positioned such that its column-vectors (a column being a group of the array's columns) are parallel to the flight direction. Forward motion compensation (FMC) is provided electronically on-chip (in the detector focal plane array) by the transferring of charge from a pixel to the next adjacent pixel in the direction of flight during the sensor's exposure time (also called “integration time”). The charge transfer rate is determined separately for each column (or for the whole array as in U.S. Pat. No. 6,256,057 where a slit is moved in parallel to the columns direction), depending on its individual distance (range) from the captured scenery, assuming flat ground. In WO 97/42659 this concept is extended to handle transferring of charge separately for each cell instead of column, a cell being a rectangular group of pixels. In the system of U.S. Pat. No. 5,692,062, digital image correlation between successive frames captured by each column is performed, in order to measure the velocity of the scenery with respect to the array, and the correlation result is used for estimating the average range of each column to the scenery, for the purpose of motion compensation in terrain with large variations. This compensation method requires capturing of three successive frames for each single image, two for the correlation process and one for the final motion-compensated frame. The system of U.S. Pat. No. 5,668,593 uses a 3-axis sightline stepping mechanism for expanding coverage of the area of interest, and it applies a motion compensation technique by means of transferring of charge along columns. U.S. Pat. No. 6,130,705 uses a zoom lens that automatically varies the camera field of view based on passive range measurements obtained from digital image correlation as described above. The field of view is tuned in accordance with prior mission requirements for coverage and resolution.        
The present invention particularly relates to an across track scanning. Typically, in such a system the focal plane array is positioned on gimbal that scans the area of interest below the aircraft from the right horizon to the left horizon and back, while periodically capturing images. Typically, there are two types of mechanisms for compensating for the aircraft movement, as follows:                a. a typical step and stare system in which the gimbals system fixes the line of sight between the focal plane array and the relevant area portion, while any aircraft movement during the integration period is compensated mechanically by the gimbals adjustment such that the line of sight remains fixed. A drawback of the step and stare system is that the gimbals progresses in a non-continuous manner, i.e., it involves a frame capturing period (integration time) during which the gimbals stops its along-track movement, followed by acceleration to a next area portion, and a next frame capturing period during which the gimbals is stationary, and so on. The discontinuity of the gimbals progression, and particularly the periods in which the gimbals is stationary (during the integration period) significantly reduces the size of the area which can be scanned by an airborne reconnaissance system in a given time.        b. A back-scanning step and stare system in which the gimbals system continuously progresses the direction of the line of sight from the left horizon to the right horizon, while this gimbals movement during the integration period is compensated by a back-scanning mirror. Specifically, the back-scanning mirror is activated during the integration period and it moves in a direction opposite to the gimbals left-right progression in such a manner that the line of sight between the focal plane array and the area of interest remains fixed. The motion compensation for the aircraft progression may be obtained in various ways, either electronically, by the same back-scanning mirror, by enabling it to have two degrees of freedom (i.e., by also providing to it movement opposite to the aircraft direction). Optionally, the along track and across track compensations may be obtained by use of two separate back scanning mirrors. This use of back scanning mirrors enables a slightly higher size of the area which can be scanned by an airborne reconnaissance system in a given time, as the across track gimbals motion from the left to the right is essentially continuous without mechanical stops during the integration periods. However, although the speed of scanning is relatively continuous a part of the cycle period between the left horizon to the right horizon, the gimbals still have to decelerate before reaching the right and left ends of the cycle respectively, up to a full stop, which follows by changing the direction of the gimbals movement from left to right, and vice versa. During this deceleration periods the size of the area which can be scanned in a given time is significantly reduced.        
As will be elaborated hereinafter, there are several factors that limit the size of the area which can be scanned by an airborne reconnaissance system in a given time, among them: (a) The row width D (in the direction of flight), i.e., the width (in the direction of flight) of the captured area portion and the altitude (above the terrain) of the aircraft, or in fact the “spanning angle” of the gimbals from the far left to the far right states; (b) The velocity V of the aircraft; (c) The maximal frame rate (i.e., the number of frames per second that can be captured, this parameter is, among others, limited by the integration period); and (d) The acceleration and deceleration periods of the gimbals system when it approaches the right and left ends.
WO 2007/004212 entitled “Method for reducing the number of scanning steps in an airborne reconnaissance system, and a reconnaissance system operating according said method” by same applicant discloses a system in which at each scanning step an image of a terrain portion which is several times larger than the size of the focal plane array is provided at the focal plane. A mirror somewhere at the optical path causes each time another section of said image to be impinged on the focal plane array. In such a manner, the number of scanning steps is reduced, as during one step plurality of frames can be obtained. However, the method and system of WO 2007/004212 still involves many stops of the gimbals, one stop for each scanning step, resulting in reduction of the size of the area which can be scanned in a given time.
It is therefore an object of the present invention to increase the size of the area which can be scanned by an airborne reconnaissance system in a given time.
It is a more specific object of the present invention to increase and maximize the size of the area which is scanned in a given time, by an across track airborne reconnaissance system.
It is another object of the present invention to obtain the above objects in a reliable and efficient manner.
Other objects and advantages of the invention will become apparent as the description proceeds.